Strong cooperations among diazotroph and arbuscular mycorrhizal fungi taxa promote free-living nitrogen fixation at soil-rock mixing layer

•Soil nutrients and diazotroph and arbuscular mycorrhizal fungi (AMF) abundance were high in the top soil;•Free-living nitrogen fixation (FLNF) activity increased from topsoil to soil-rock mixing layer;•Increased FLNF activity via strengthen cooperation between diazotroph and AMF taxa;•Diazotroph and AMF keystone taxa were the key predictor of biological nitrogen fixation.